1. Field of the Invention
The present invention relates to a fast reservation protocol (FRP) type asynchronous transfer mode (ATM) switching process, and particularly to an ATM switching method and an ATM switching equipment which can efficiently store receiving ATM cells in a buffer and can transmit them.
2. Description of the Related Art
A fast reservation protocol (hereinafter referred to as FRP) adopts a transmission system in which ATM cells through an ATM subscriber's line are transmitted consecutively as in a burst mode, as shown in FIG. 3(A). An ATM cell burst is composed of a resource management cell (RM cell) arranged at the top of the burst stream and the succeeding plural cells. The RM cell contains an attribute information of the burst, such information as the total number of cells in the burst, and the information of the total number of cells in the burst is utilized for control of storing the cells in a buffer in the ATM switching equipment.
FIG. 4 is a block diagram showing a fundamental composition of an FRP type ATM switching equipment according to the prior art. In FIG. 4, the FRP type ATM switching equipment is mainly composed of an ATM subscriber's line 21, an RM cell processing circuit 22 which is comprising a control circuit 26 and a gate circuit 27 for processing an RM cell received through the ATM subscriber's line 21, a multiplexer circuit (MUX) 23, a buffer 24 for storing ATM cells to be transmitted, a buffer monitoring circuit 25.
Operation of the above-mentioned ATM switching equipment is described in the following with reference to FIG. 3(A).
First, a receiving RM cell (1) is received by the ATM switching equipment. In the ATM switching equipment, the control circuit 26 of the RM cell processing circuit 22 extracts information of the total number of cells in the ATM cell burst being arrived which is an attribute information stored in the receiving RM cell (1). On the other hand, the ATM switching equipment always monitoring its own receiving capacity available by the buffer monitoring circuit. The buffer monitoring circuit 25 detects number of available idle slots in the buffer 24 and informs the detected result to the control circuit 26 of the RM cell processing circuit 22. Then, the control circuit 26 compares the extracted information, the total number of cells in the ATM cell burst being received, with its own receiving capacity, number of available idle slots in the buffer 24, informed by the buffer monitoring circuit 25. When the comparing result indicates that the buffer 24 has enough capacity to store all the receiving cells (2) to (7) in the receiving or incoming ATM cell burst, the control circuit 26 opens the gate 27 to store the receiving cells in the buffer 24 through the multiplexer circuit 23. However, when the comparing result indicates that the buffer 24 does not have enough capacity to store all the receiving cells, the control circuit 26 discards all of the receiving cells (2) to (7) by closing the gate circuit 27.
In such a way, a receiving method of receiving cells in an existing switching equipment has performed an alternative control which is either of reception and discarding of received cells according to relation between the total number of ATM cells in the burst and the number of idle slots available in the buffer.
In a conventional FRP type ATM switching equipment, in the case that the total number of cells in a receiving ATM cell burst is great, the possibility that the number of idle slots available in the buffer is greater than the total number of cells in the burst becomes low, and as the result, discarding of a receiving ATM cell burst is liable to happen.
In the case that the total number of cells in an ATM cell burst to be received is greater than the number of idle slots in the buffer, all cells in the burst are discarded. Therefore, the transmission side of the ATM cell burst needs to retransmit the same ATM cell burst, and this causes a discarding rate of cells to increase and also causes the transmission efficiency to be deteriorated.
Particularly, in the case of transmitting data having a great amount of information, since a huge ATM cell burst is processed, it is likely that a high-speed data transmission cannot be performed due to discarding of cells.